Heating through fractures to expand a shale oil pyrolyzing cavern

ABSTRACT

In a process for recovering shale oil by injecting and producing fluid into and out of a rubble-containing cavity in an otherwise substantially non-porous and impermeable subterranean oil shale formation, hot gas is circulated into and out of the cavity through spaced horizontal fractures to preheat the oil shale formation and increase the rate of oil recovery and cavity expansion.

0 United States Patent m: [In 3,888,307

Closmann June 10, 1975 HEATING THROUGH FRACTURES T0 22 3413; gzpagopoulset al log/9297,:

IN ar I... A SHALE OIL PYROLYZ G 3.759328 9/1973 Ueber l66l303 3.759,5749/l973 Beard A 299/4 [75] Inventor: Philip J. Closmann, Houston, Tex.3.804,l69 4;!974 gosmann {665567 3,804,l72 4 I974 mann 66 72 [73]Assignee: Shell Oil Company, Houston Tex. 0s

[22] Filed: Aug. 29, 1974 Primary Examiner-James A. Leppink 2| A I. No.:501 535 I pp 57 ABSTRACT In a process for recovering shale oil byinjecting and 5 producing fluid into and out of a rubble-containingcavity in an otherwise substantially non-porous and [58] held Search66/27 I 299/4 impermeable subterranean oil shale formation, hot gas iscirculated into and out of the cavity through spaced [56] ReferencesCited horizontal fractures to preheat the oil shale formation UNITEDSTATES PATENTS and increase the rate of oil recovery and cavity expan-3,386,508 6/!968 Bielstcin et al. 166/272 Sign, 3,468,376 9/l969 Slusseret al loo/272 3339,85: 6/1973 Beard :66/254 4 Chums, 1 Drawing lgurHEATING THROUGH FRACTURES TO EXPAND A SHALE OIL PYROLYZING CAVERNBACKGROUND OF THE INVENTION The invention relates to producing shale oiland related mineral materials from a subterranean oil shale formation.

Numerous subterranean oil shale formations contain water-solubleminerals and/or minerals that are thermally converted to water-solublematerials. Such oil shales, in their natural state, are non-porous,impermeable aggregations of solid organic and inorganic materials. Asindicated by patents such as the T. N. Beard, A. N. Papadoupolos, R. C.Ueber US. Pat. Nos. 3,739,85 l; 3,741,306; 3,753,594; 3,759,328 and 3,759,574, shale oil can be recovered from such formations by utilizing thewater-soluble and/or heatsensitive materials to form relativelysolids-free rubblecontaining caverns within which the oil shale iscontacted by a circulating hot aqueous fluid that disaggregates the oilshale into fluid-surrounded particles, con vcrts the kcrogen to fluidhydrocarbon materials, and dissolves or entrains enough inorganic solidto expand the cavern while producing shale oil. In such prior processes,the transferring of heat from an injected hot fluid such as steam to theoil shale formation is aided by inflowing the fluid into an upperportion of the cavity and withdrawing fluid from the lower portion ofthe cavity. However, as discussed in the P. J. Closmann and G. O. SumanUS. Pat. Nos. 3,804,|69 and 3,804,172, the processes are subject to atendency for such bottom-out" flow paths to become plugged due to thefilter-cake-forming tendency of a fluid containing a significantproportion of fines mixed with larger particles. Where, for example, thehot aqueous fluid is the condensate from a top-injected steam, thecondensate flows down along the walls of the cavern while rubblizing anddisaggregating portions of the oil shale. This entrains or causes theslumping of particles having sizes ranging from only a few microns toseveral feet in diameter. Such particles, which slump or flow inresponse to gravity as lumps or a turbidity current moving down thewalls of the cavern, tend to pile-up and plug-up around thefluid-withdrawal point near the bottom of the cav- In the above US. Pat.No. 3,804,l69, a pattern of fracture-interconnected caverns is equippedwith wells arranged so that the hot fluid injected near the top of onecavity is produced through a plurality of surrounding cavities at flowrates arranged so that the flow velocities decrease with radial distanceto velocities that are too low to carry the solids to the producingwells. However, where an oil shale formation is thin enough to causesignificant amount of heat loss to other formations, the so-arrangedflow rates may be uneconomically slow. In the above US. Pat. No.3.804,l72, the lower portion of such a rubblecontaining cavity is packedwith a mass of relatively large rigid solid parti cles, or boulders, sothat the slurried solids are spread out over large surface areas whilethe fluids are flowing through the relatively large openings that existbetween such particles.

In copending patent application Ser. No. 489,639, filed July l8, 1974 byP. J. Closmann and M. J. Tham, the tendency for the flow path to becomeplugged in such a shale oil recovery process is reduced by injectingboth a hot solvent-fluid and a hot non-solvent-gas at rates correlatedso that the cavern remains substantially free of liquid. However, inusing fluids such as steam and nitrogen, this has a tendency to slow therate of oil production and cavern expansion. Increasing the proportionof steam increases the rate of fines production, and decreasing theproportion of steam decreases the rate of oil production and cavityexpansion.

SUMMARY OF THE INVENTION The present invention relates to an improvementin a process in which shale oil is produced by forming a relativelysolids-free rubble-containing cavity or cavern within an otherwisesubstantially non-porous and impermeable subterranean oil shale thatcontains water soluble minerals, injecting fluid into the cavern,withdrawing fluid from the cavern and recovering shale oil from thefluid withdrawn from the cavern. The improvement comprises thefollowing. At least two verti Cally-separated generallyhorizontally-oriented fractures are extended between a cavern and atleast one laterally displaced well. A hot non-solvent-gas having arelatively insignificant miscibility with any of the organic orinorganic components or pyrolysis products of the oil shale is flowedinto the cavern through at least one of the fractures while a hot fluidinclusive of oil shale pyrolysis products is flowed out of the cavernthrough at least one other horizontal fracture. The rate and temperatureof the fluid inflowed through a fracture and the rate of the fluidoutflowed through the fracture are adjusted so that the fracture wallsare heated and become vertically separated, generally planar heatsources from which heat is transferred to the oil shale formation beyondthe walls of the cavity.

DESCRIPTION OF THE DRAWINGS The drawing schematically illustrates aportion of a subterranean oil shale formation in which the presentinvention is being practiced.

DESCRIPTION OF THE INVENTION The present invention is, at least in part,premised on the following. In an oil shale recovery process in which ahot fluid is injected into and produced from a relatively solids-free,rubble-containing cavern with a subterranean oil shale that is otherwisesubstantially nonporous and impermeable, the heat transferringefficiency is improved by a circulation path in which the point ofinjection is near the top of the cavity and the point of withdrawal isnear the bottom of the cavity. The plugging of such a flow path due tothe entraining and screening-out of particles by a hot solvent-fluidthat is relatively miscible with one or more of the organic or inorganicsolid components or pyrolysis products of the oil shale can be reducedwithout significantly reducing the rate of oil shale recovery or caverngrowth. The kerogen is such an oil shale can be pyrolyzed while formingrelatively few fines by contacting the oil shale with a hotnon-solvent-gas that is substantially immiscible with any of the organicor inorganic solid components of the oil shale or oil shale pyrolysisproducts. The rate of non-solvent-gas-induced pyrolysis can be maderelatively rapid by using temperatures in the order of 700 to lOOOF. Therate of the heat transfer from the cavity to the surrounding oil shaleformation can be materially enhanced by forming vertically-separatedgenerally-horizontal fractures and circulating such a hotnon-solvent-gas in through one while circulating a hot gas out throughanother. This, in effect, surrounds the cavity with a pair ofradiallyextensive, generally planar, heat sources. It causes the oilshale formation around the cavity to be caught between the jaws of athermal-pincer and rapidly heated to a temperature at which thegas-pressure-induced spalling of fragments of oil shale into the cavitycauses a relatively rapid expansion of the cavity.

As the terms are used herein, oil shale refers to a substantiallynon-porous impermeable aggregation of inorganic solids and apredominately hydrocarbon solvent insoluble inorganic solid materialknown as kerogen. Bitumen" refers to the hydrocarbon solvent solubleorganic material that may be present in the natural oil shale and isusually the initial thermal conversion or pyrolysis product of kerogen.Shale oil refers to gaseous or liquid hydrocarbon materials, which maycontain trace amounts of other elements such as nitrogen, sulfur, oxygenor the like, that can be obtained by pyrolyzing and/or extractingorganic materials from an oil shale. Water soluble inorganic minerals"refers to those which are naturally water-soluble solids (having asolubility of at least about grams per 100 grams of solvent in aqueousliquids having pHs of from about 5 to 8) such as the halites orcarbonates typified by the alkali metal chlorides, bicarbonatcs, orcarbonates, as well as the heat-sensitive minerals that are eithernaturally water-soluble or are thermally converted to materials that arewater-soluble, such as nahcolite, dawsonite, trona or the like minerals.A water-solublemineral-containing subterranean oil shale is an oil shaleformation that contains, or is mixed with, at least one water-solublemineral in the form of lenses, layers, dispersed particles or the like.

The present process can be used in substantially any relativelysolids-free cavity or cavern or opening in an otherwise substantiallynon-porous, impermeable subterranean oil shale. Such a cavern ispreferably at least as large as the borehole of a well, has a relativehigh permeability such that the rate of gravity seggregation of fluidsis not significantly impaired by a lack of permeability, has a porosityof from about to 95%, and is substantially free of interconnected solidsthat are ridgidly connected to the matrix of the surrounding earthformations. As indicated in the previously mentioned Beard et alpatents, such caverns can be formed by solution and/or mineral mining,nuclear detonation or other explosive fracturing coupled with the miningout of a significant portion of solids. Such caverns are advantageouslyformed by solution-mining an opening in awater-soluble-mineral-containing subterranean oil shale in which thereare vertically separated layers of water-soluble mineral such as haliteor nahcolite.

The fractures used in the present process can be substantially any thathave generally horizontal and radially extensive permeable channels,such as hydraulically-induced fractures that are propped open withgranular propping materials, or fractures having walls that have beenetched to provide channels that remain open, or the like. Such fracturescan advantageously be formed by locating vertically separated layers ofsoluble material such as a halite, inducing a fracture along a boundaryplane of such a material and etching the material by solution-mining toprovide a generally radially extensive self-supporting fracture flowchannel. Patterns of cavities and/or wells are preferably arranged sothat centrally located wells, such as the center wells in the five-spotpattern of wells and/or wellcontaining cavities can be used to circulatefluids into and out of fractures that extend into each of a series ofsurrounding cavities.

A hot solvent-fluid suitable for use in the present process is one whichis heated to a temperature of about 500 to 700F and,'at such atemperature, exhibits a significant miscibility with at least one of theorganic or inorganic solid or liquid pyrolysis products of awatersoluble-mineral-containing oil shale. Such fluids preferablycontain (or consist essentially of) steam at a temperature and pressurecausing condensation within the cavern. Such fluids may also include orcomprise hydrocarbons such as benzene, toluene, shale oil hydrocarbons,oil-soluble gases such as carbon dioxide, mixtures of such fluids, orthe like.

A hot non-solvent-gas suitable for use in accordance with this inventioncan comprise substantially any gas having a temperature of at leastabout 500F and, at that temperature, has a relatively insignificantmiscibility with any of the organic or inorganic solid or liquidpyrolysis products of a water-soluble-mineralcontaining oil shale (e.g.having a solubility of less than about 1 part per thousand in such solidor liquid pyroly sis products). Suitable non-solvent-gases include nitrogen, natural gas, Combustion gases, methane that is substantially freeof higher hydrocarbons, mixtures of such gases, and the like.Particularly where steam is used as (or as a component of) the hotsolventfluid, the hot non-solvent-gas can be injected at temperatureshigher than about 700F, for example, to enhance the rate of revaporizingthe steam condendate and the drying out of the cavern.

As shown in the drawing, a series of wells, 1, 2, and 3, are completedinto a subterranean water-soluble mineral-containing oil shale formation4. In well 2, an outer conduit 6 is terminated near the upper portion ofthe oil shale formation, while an inner conduit 7 extends to asignificantly lower depth. A relatively solidsfree cavity 8, containinga rubble of oil shale particles 9, is formed, for example, by solutionmining techniques. Generally horizontally oriented fractures 11 and 12are formed along layers of a water-soluble mineral such as halite ornahcolite contained with the oil shale formation. The fractures can beformed by conventional hydraulic fracturing techniques and propped oretched to maintain a permeable channel between their walls. Thefractures can be formed prior to or after the expanding of a borehole tothe size of the cavity 8 (which expanding can be accomplished by, forexample, installing conduits 6 and 7 as shown and circulating in a hotaqueous solvent through conduit 6 while circulating out through conduit7 a solution of watersoluble minerals).

Wells 3 and 4 are completed in locations into which the fractures ll and12 can be extended. They are preferably completed with an outer conduit13 and an inner conduit 14 with a packer 16 positioned near but abovethe depth of the depth of the fracture l2 and the end of the conduit 14.The outer conduits 13 are preferably perforated with perforations suchas 17 and 18 near the depths of the fractures 11 and 12.

In a preferred operating procedure, the fractures 11 and 12 are extendedbetween the wells 1, 2, and 3 prior to any solution mining or cavityexpanding around well 2 to extend the bore hole beyong a diameter atwhich packers can be utilized to facilitate fracturing at at least apair of selected depths. An aqueous liquid is circulated through thefractures ll and 12 (sequentially or simultaneously and/or by means offlow patterns that are periodically reversed) to cause the growth andextension of the etched portions of the fracture walls. The cavity 8 isthen advantageously formed by a solutionmining procedure, for example,such as the one described in the L. H. Towell, .I. R. Brew U.S. Pat. No.3,792,902. When the cavern has dimensions in the order of a 100 to 200foot diameter and 300 to 500 foot height, the hot-gas-aided process ofthe present invention is preferably initiated.

A hot-non-solvent-gas, such as nitrogen, is circulated in through wells1 and 3 to flow into the cavern 8 through fractures ll while a hotgenerally gaseous fluid is outflowed through fracture 12. In the initialstages, it is advantageous to heat up all of the cavern relativelyrapidly by concurrently circulating a similar hot gas into cavern 8through conduit 6, while fluid (which is initially apt to be liquid orinclude liquid) is circulated out through conduit 7.

The inflowing hot gas can be substantially any nonsolvent gas heated toa temperature in the order of 700 to l000F by means of surface and/ordownhole located heating devices. The outflow conduit 7 in cavity 8 isadvantageously extended to a depth below that of the lowermosthorizontal fracture 12. This ensures that most of any liquid thatoutflows from the cavern will be withdrawn from the conduit 7 and willtend to reduce the plugging of fracture 12 by solids that may beentrained in an overflowing liquid (which would have a velocity thatwould decrease with radial distance away from the cavity 8).

Where it is desired to enhance the rate of oil shaledisaggregationand/or solid removal by solution or entrainment of solids in a liquid.the circulation of the hot non-solvent gas in through fracture 11 andout through fracture 12 can be continued while a selected propor tion ofa solvent-fluid such as steam and/or hot aqueous liquid is concurrentlycirculated in through conduit 6 and out through conduit 7. Theproportions of the circulating hot non-solvent and solvent fluids arepreferably controlled so that the cavern is kept substantiallyliquid-free at least to an extent that prevents any significant amountof plugging relative to the outflow of fluid through conduit 7. Aninclusion of a suitable, but not excessive, proportion of such a hotsolvent-fluid can readily be attained by establishing a selected rate ofcirculation of hot non-solvent fluid in through conduits 6 and 13 andout through conduits 7 and 14 and then including increasing proportionsof solvent fluid in the stream inflowing through conduit 6 whileobserving any tendency for a significant decrease to occur in the amountof fluid outflowing through column 7.

The present process is particularly suitable for use in anahcolite-containing subterannean oil shale. In such a formation, thedecomposition of the nahcolite (by heating it to a temperature at whichit is converted to CO, and a water-soluble carbonate) is an effectivemechanism for inducing the rubbling of the oil shale formation. When thewalls of a cavity are heated to a sufficient temperature. the pressurewhich is built up within the rock by the CO being generated by thethermal conversion of the nahcolite (which is commonly present indiscontinuous nodules within the oil shale formation), tends to createfractures and to hydraulically displace portions of the oil shale towardthe relatively solids-free void space within the cavity. This has beenshown to be quite effective in expanding a cavity into which steam isinjected while a liquid solution of dissolved inorganic material isbeing outflowed. The dissolving of the carbonates (into which thenahcolite is thermally converted) increased the pH of the steamcondensate and this increases the rate of disaggregation of the oilshale that is contacted by the then alkaline aqueous liquid. This,however, has a disadvantage of increasing the tendency for the pluggingof the flow path and for being most effectively conducted at arelatively low temperature, of less than about 700F. Since an oil shalerock formation is generally a poor conduc tor of heat, and since theheating or a rock builds up thermal stresses that might inhibit thefracturing of the rocks around a cavity and/or nodules of nahcolite,tests were made and model studies were made, to determine the overalleffect of these stresses. In order to fracture the rock and cause therubbling, the gas pressure must overcome the tensile strength of therock and both the induced thermal stresses and the in situ horizontalstress within the earth formation at the depth being considered.

By subjecting the oil shale beyond the fracture wall to a more rapidheating, i.e. by subjecting it to the heat being conducted away from apair of generally planar heat sources (i.e. the hot gas conveyingfractures above and below such portion) the rate of cavity growth willbe materially increased. In the present process, the spacing of suchhorizontal fractures is preferably such that there is at least about onepair per each lOO to 200 feet of vertical height of the cavity.

What is claimed is:

1. In a shale oil producing process in which a relatively solids-free,rubble-containing cavity is formed within an otherwise substantially nonporous and impremeable subterranean oil shale, and fluid is injectedinto and withdrawn from the cavity, the improvement which comprises:

forming at least two vertically-separated, substantially horizontalfractures that extend between the cavity and at least one adjacent well;

inflowing a hot non-solvent-gas into the cavern through at least one ofsaid fractures;

outflowing a hot fluid containing oil shale pyrolysis products from thecavern through at least one other of said fractures; and

adjusting the rate and temperature of the fluid inflowing and outflowingthrough the fractures so that the fracture walls are heated and becomevertically separated, generally planar heat sources from which heat istransferred to portions of the oil shale formation surrounding the wallsof the cavity.

2. The process of claim 1 in which said fractures are located along theboundaries of layers of water soluble minerals that are present withinsaid oil shale forma tion.

3. The process of claim 1 in which said circulation of hot non-solventgas is continuously or intermittently supplemented by a circulation of ahot solvent fluid into the cavity through a conduit terminating in anupper portion of the cavity and a circulation of fluid out of the cavityfrom a conduit terminating in a lower portion of the cavity.

4. The process of claim 3 in which the hot nonsolvent-gas is nitrogenand the hot solvent-fluid is steam.

1. IN A SHALE OIL PRODUCING PROCESS IN WHICH A RELATIVELY SOLIDS-FREE,RUBBLE-CONTAINING CAVITY IS FORMED WITHIN AN OTHERWISE SUBSTANTIALLYNON-POROUS AND IMPREMEABLE SUBSTERRANEAN OIL SHALE, AND FLUID ISINJECTED INTO AND WITHDRAWN FROM THE CAVITY, THE IMPROVEMENT WHICHCOMPRISES: FORMING AT LEAST TWO VERTICALLY-SEPARATED, SUBSTANTIALLYHORIZONTAL FRACTURES THAT EXTEND BETWEEN THE CAVITY AND AT LEAST ONEADJACENT WELL; INFLOWING A HOT NON-SOLVENT-GAS INTO THE CAVERN THROUGHAT LEAST ONE OF SAID FRACTURES; OUTFLOWING A HOT FLUID CONTAINING OILSHALE PYROLYSIS PRODUCTS FROM THE CAVERN THROUGH AT LEAST ONE OTHER OFSAID FRACTURES; AND ADJUSTING THE RATE AND TEMPERATURE OF THE FLUIDINFLOWING AND OUTFLOWING THROUGH THE FRACTURES SO THAT THE FRACTUREWALLS ARE HEATED AND BECOME VERTICALLY SEPARATED, GENERALLY PLANAR HEATSOURCES FROM WHICH HEAT IS TRANSFERRED TO PORTIONS OF THE OIL SHALEFORMATION SURROUNDING THE WALLS OF THE CAVITY.
 2. The process of claim 1in which said fractures are located along the boundaries of layers ofwater soluble minerals that are present within said oil shale formation.3. The process of claim 1 in which said circulation of hot non-solventgas is continuously or intermittently supplemented by a circulation of ahot solvent fluid into the cavity through a conduit terminating in anupper portion of the cavity and a circulation of fluid out of the cavityfrom a conduit terminating in a lower portion of the cavity.
 4. Theprocess of claim 3 in which the hot non-solvent-gas is nitrogen and thehot solvent-fluid is steam.